Frequency modulation receiver tuning aid



' -Mrch 13,1945. w. R. KOCH f 2,371,397

FREQUENCY MODULATION `RECEIVER TUNING AID FilelMayl, 1942 -T'lci FIMDsrfcm/Pf l l y --L 1 l Ltllln N Il j?, '1:15p IfT ATTORNEY Patented Mar. 13, 1945 UNITED .STATESl PATENT OFFICE FREQUENCY MoDULATIoN RECEIVER TUNING An) Winfield R. Koch, Haddonfeld, N. J., assignor to v Radio Corporation of America,

Delaware a corporation of Application May 1, 1942, Serial No. 441,298 11 Claims. (Cl. Z50-20) My present invention relates to improvements ink tuning aids for modulated carrier wave receivers, and more particularly to circuits for providing improved receiver .tuning and reduced inter-station noise reproduction.4

In receiving amplitude modulated v(AM) carrier waves it is customary to tune the receiver, in the absence of a visual indicator, by listening to the audio output. Accurate tuning is indicated by maximum sound output; the volume falling on. appreciably on either side of resonance. However in the case of the reception of angular velocity-modulated carrier waves, such as frequency modulated (FM) waves, uniform sound output is obtained over a considerable range with respect to a gillen mid-band, or center, frequency. The

Another important object of my invention is ,tol

provide in combination with an FM detector having a predetermined operating frequency corresponding to the center, or mid-band, frequency of a desired station; a device for rendering 'ineffective transmission through the receiving system in response to the mean frequency of applied FM Waves differing from said operating frequency.

Still another important object of my invention relates to a receiver adapted to receive mod-` ulated waves of a variable frequency; an audio amplifier being employed to amplify the detected waves, and means being utilized to cause audio amplification to be operative solely when the re` ceiver is tuned to the center, or mid-band, frequency of waves of a desired station.

' Yet other objects of my invention are to improve the tuning of FM receivers, and more especially to. provide tuning aids for A.Flvli receivers which are durable, reliable in operation and economically manufactured and assembled.

The novel features which I believe to be characteristic of 'my invention are set forthwith particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the 'following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing:

Fig. 1 shows a circuit diagram of one form of the invention,

Fig. 2 illustrates a modification,

Fig. 3 shows a variation of the arrangement of Fig. 2.

Referring now to the accompanying drawing, wherein like reference characters in the different figures designate similar circuit elements,- there is shown in Fig. 1 only so much of an FM receiver Which is necessary to a proper understanding of the present invention. Those skilled in the art are fully aware of the fact that in FM broadcast reception the superheterodyne type of reception is employed. The FM waves cover an assigned band of 42-50 megacycles (mc). The channel width for each FM station is 200 kilocycles (kc). Actually, however, the FM transmitter deviates the carrier frequency up to a maximum of kc. to either side thereof in accordance With the amplitude of the modulation signals. Of coure, the rate of carrier frequency deviation is determined by the frequencies of the modulation signals themselves. The present invention is not limited to the 42-50 mc, band, nor is it limited to the reception of FM waves. The term angular velocity-modulated carrier waves used herein generically is to be understood as covering'phase modulated or frequency modulated carrier waves.

The collected FM waves are amplified in one or more stages of tunable radio frequency amplication prior to impression of the collected waves upon the tunable input circuit of the first detector or converter stage. As is well known, the local oscillator has its tank circuit tunable over a range of local oscillation frequencies which differ from the signal frequency range by the value of the operating intermediate frequency (I. FJ. The latter may be chosen from a range of 4 to 20 mc.; as for example, 4 mc. The I. F. energy is amplified in one or more I. F. amplifiers. In order to minimize amplitude variations that may exist on the modulated carrier waves, a limiter stage is usually employed prior to detection of the amplified I. F. energy.l y

Of course, each of the tuned transmission networks lin the receiving system will have a pass band of 200 kc. so as to pass efficiently the modulated carrierl waves applied to each stage, It is to be understood that the numeral I designates the resonant I. F. circuit connected to the plate of the limiter tube, or to the plate of the last I. F. ampliiier tube if no limiter is employed. The detector tube itself is designated by numeral 2, and may be a double diode rectifier of the BHS type. The anodes vof the two diodes are connected Seeley in his U. S. Patent No. 2,121,103 granted It is believed suilicient for the June 21, 1938. purposes of thisv patent application to point out that `the function of ,the discriminating network I-'Il-S is to derive from the wai/es 'of constant amplitudejjand variable frequency waves fof variable amplitude. In other words, the waves applied to the circuit I are applied to the anodes of the opposedA diodes with an 'amplitude variation representative of the vinddulatio'n of the r'eceived carrier wavest The cathodes of the opposed diodes are 'connected in series by resistors and B, the cathode end of resistor I being connected to ground by an audio frequency by-pas's' condenser "L The resistors `5 and 76, which are the load resistors 'of the detectonlare each luy-passed by condensers 8 and!! respectively for I. Rolli-rents. The modulation signal 'voltage developed across the load resistors 5 and 6 is applied 'to the control grid of the audio frequency zamplifier tube IB. The choke coil l3' connects the junction 'of E14-'S to the midpoint of the coil of circuit 3 thereby providing a return lpath for the diodes. y

The grid Il of audi-o amplifier ytube 'I0 yis co`nnected to the cathode fend of "resistor `5 by the audio coupling "condenser 12. The cathode of tube Ill is connected to'lgrufid by thegrid biasing network I3. sries 'resist-.ors 14 and l5 are connected from grid Il t'o grou'd. vAn Yaudio A-lltei network couples the electrodes of the mutlng diodes to the load resistors 5-'B. Thus, 'diodes I6 and I'l are arranged in opposition; the ycathodes thereof being connected in common to ground. The vanodes'of diodes 'I6 and I'I are connected to kassociated ends of lresistors 5 and G through the respective-filter resistors 5' and B'. Diodes I6 and I'I are shunted by respective iilter condensers IB'and I'I.

A diodey I8 has its cathode connected t0 the anode of diode I6, while its anode is connected tothe junction of Aresistors I4 and i5. A diode I9 has its anode connected to the anode of diode I 8, while the cathode ofthe former is connected to the anode of diode I'I. The plate circuit of audio amplifier I0 vmay be coupled to further stages of audio amplification, and the latter may terminate inranyI desired form of reproducer.

Before explaining the specific operation of my invention, it is explained that accurate tuning of the system results in having the I. F. energy applied to discriminator I-,3 properly centered about a mid-frequency which is equal to the operating Value of the I. F. Those skilled yin the a-rt realize that the Srl-shaped ,discriminator curve is symmetrical relative to the center frequency (the I. F. value) and that if the midfrequency of the applied signal energy shifts relative lto :said

center frequency there may result high distortion and noise. Further, since there is applied to grid II the resultant of the voltages developed across resistors 5 and 6, these voltages are equal and of opposite polarity when the applied signal energy has a center frequency equal to the resonant frequency of circuits I-3. If the center frequency deviates from .the said vresonant frequency, as would be the case in detuning in a given station channel, voltage will be developed which can be used in the manner now to be described.

lWhen the instantaneous frequency of the applied I. F. energy is equal to the operating I. F. value, which is thefrequency of circuits I and 3,

.there will be no effect on the bias of the audio' As a result thegr'id I YI will be biased ififreasi'glyA negative, and the gain of 'tube I is substantially reduced. A

If, now, the mistunin fi" tion, the other twodides I'I 'a-rx'd I8 through the `'path I'il-gnb'uid; SLW-1S'. Here, again, negative 'bias vi-s developed fa'ciiis's resistor 'I5 Vfor reducing 'the `gain of tube TU. iB'y using a sharp cut-oir tube at I0, the latter can be completely cut oi for oi-tune positions rof the tuning -device thereby mak-ingv sure 'that th win not tune the receiver 'to the spaced'ofrztue peaks of the S-shap'ed discriminator characteris tic where the distortion is great. fsun silencing action for oli-tuning will, also, prevent the vlllsei' from tuning to theoutside slopes of the x'disc'iine inator characteristic where thefnise llei're'lisli-ign.- Evn if the'gafn 'of tube l0 'is reduced instead 0f the tubebeihg cut iof', f'o'r the condition of'i'nis# tuning vthe 'set u'ser Will be :able accurately "to the receiver, 'because lwill fgiv'e hirn the 's-trie volume output variation as when tuning an arn` plitudemodulated set.

' Of course, the invention i'sfnot l'iinite'd to the precise type of disciir'niifato'r For exampletliie may lbe 'used any weil known form 'df M. tie'-l tector which generally compri-ses opposed 'rect-.ie fiers fed through =`oppositely sloping "lt'ers Whose cross-over point is the 'center `fre'tlun'c'iy of applied waves. Further, four diodes need not be used. In other words, there may be used the two diodes of Fig. t2; in tha-tease less biasing vol'tge is developed "for the audio ampliiier `tube. l

In Fig. 2 diodes VI6 and "Il `are mploye'dfwhile diodes I8 land vI9 are `dispensed with. Resistors- 20 and Y2l are 'shunted across respective fdiod'es .fI'Gv and I'I. The resistors l2l) Vand 2I act as 'fgrd Iealc resistors for the 'grid II\. Audio ampli-lier, I'Il" may be of the inultiegrid ltype. Signal grid I-I. has the modulation signals applied thereto. Gain control is elfected by having grid II' lconnected to the junction of yresistors I4 and +5. Gain control bias may 'be applied, through vfilter 're'.e sistor 22, to the control grid of -a iprior AI. EF. iamplifler which follows. the limiter. If desired, the bias may be applied to the 'control grid of the, limiter to reduce Vthe output level 'of the limiter for off -tuning In Fig. 3, the varrangement of Fig. -2 is shown.

vmodified to the extentl of having diodes I6 and l1 reversed with the common anode connection to .the junction of resistors I4 and l5. Also, the junction of resistors |74 and I5 is connected through a grid return resistor to the control grid of audio amplifier lll, as in Fig. l.v In Fig. 2 one diode shorts the positive end of resistors 2li- 2l lto ground, leaving the negative end connected to the grid through resistors I4' or l5. In Fig. 3 one diode shorts the negative end of the resistors 2ll2| to the grid leaving the positive end connected to ground through resistors 20 or v2|. In all three forms of theinvention herein disclosed any unbalance in the direct .current `output o f thediscriminator .is rectified, and used to reduce the gain and audible output of the receiver.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim ist 1. A method of reception which includes detecting frequency modulated waves to produce Aquency value, producing solely a constant polarity direct current in response to said voltage, and reducing to a substantial extent the volume of reproduction of the modulation signals with a direct current voltage solely derived from said constant polarity current.

2. In a frequency modulated carrier wave receiver, including means for rendering the output amplitude substantially independent of the carrier amplitude; a method of reducing the output amplitude in accordance with the amount of mistuning of the receiver, including deriving a direct current which varies in polarity with the direction of mistuning, translating said direct current into solely a direct current potential of constant polarity but varying in amplitude with mistuning, and controlling solely with said potential the gain of one or more amplifier stages of the receiver.

3. In a frequency modulated carrier receiver which is not responsive to variations in amplitude of the carrier; a method of securing response to mistuning, which includes deriving a direct current of either polarity in mistuned condition, translating said direct current into solely a constant polarity direct current, deriving a'constant polarity potential from said constant polarity direct current, and controlling the receiver output i level with solely said constant polarity potential.

4. In a receiver of angular velocity-modulated carrier waves free of amplitudevariations; the

method of reducing the receiver volume in accordance with mistuning ofthe receiver which includes producing a unidirectional current whose polarity varies in accordance with the sense of mistuning, deriving from said current solely a direct current potential of constant polarity but Whose amplitude varies with mistuning, and controlling the receiver gain with solely said potential.

5. In a frequency modulationA receiver, the method of audibly indicating mistuning of the receiver which includes deriving from received waves a direct current whose polarity is dependent upon the sense of mistuning, deriving from transmission through the receiver in a the current solely-a direct current voltage ofcon,- stant polarity andvariable amplitude, and controlling the receiver' output amplitude` in response to solely the said voltage. 1

6. In a receiver of frequency modulated car.- rier waves, means for `detecting said waves to produce modulation signals ,and a direct current whose magnitude is dependent upon the departure of the mean frequency of said wavesfrom a predetermined reference frequency, at least two opposed diode rectifiers operatively associated with the detecting means f or deriving a direct current voltage of constant polarity and Avariable amplitude from said direct current, and means responsive to solely said voltage for controlling predetermined sense.

7. In a frequency modulation receiver provided with a demodulator having a pair of load resistors through which flow direct currents of polarity dependent upon the sense of mistuning of the receiver, means for utilizing the demodulated wave energy, at least two diodes arranged in opposition, said two diodes being connected across said resistors whereby said direct currents may ow therethrough, a resistive load common to the space current path of each diode for developing from said currents a direct current voltage of constant polarity but variable amplitude, and means controlling said utilization means with solely said voltage.

8. In a receiver of frequency modulated carrier waves provided with a detector capable of PrO- viding 'a direct current voltage whose polarity is dependent upon the direction of frequency shift of the waves from a reference frequency; means to provide a muting voltage comprising at least two diodes arranged in polarity opposition, means for establishing the common junction of like electrodes of the diodes at an invariable potential,

means for applying said direct current voltage to a respective one of the diodes depending upon the polarity of the voltage, a load element in circuit with each diode for developing a constant polarity direct current voltage from diode space current, and means for utilizing said last voltage as said muting voltage.

9. In a receiver of frequency modulated carrier waves provided with a detector -capable of providing a direct current voltage whose polarity'is dependent upon the direction of frequency shift of the Waves from a reference frequency; means to provide a muting voltage comprising at least two diodes arranged in polarity opposition, means for establishing the common junction of like electrodes of the diodes at an invariable potential, means for applying said directcurrent voltage to a respective one of the diodes depending upon the polarity of thevoltage, a load element in circuit With each diode for developing a constant polarity direct current voltage from diode space current, means for utilizing said last-vo1tage as said muting voltage, and additionalmeans for utilizing said last voltage for control of the re.- ceiver gain prior to said detector.

10. In a receiver of frequency modulated carrier Waves provided with a detector capable of,

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